But long before there was time-lapse photography, people were aware of, and often fascinated by, plant movements. And, especially in the 19th century, such plant movements became the subject of methodical scientific inquiry. Particularly by someone more well-known for his theory of evolution.

In this book, the Darwins report observations of a wide range and variety of movements in plants. In their own words: “The chief object of the present work is to describe and connect together large classes of movement, common to almost all plants. The most widely prevalent movement is essentially of the same nature as that of the stem of a climbing plant, which bends successively to all points of the compass, so that the tip revolves.” (See the YouTube video above for examples.)

The Darwins named this movement “circumnutation“, which is defined by the Oxford English Dictionary as: ” A movement characteristic of growing plants, due to increased growth at different points round the axis in succession, whereby the growing part (e.g. the apex of a stem) describes a more or less circular spiral path.”

The scope of their observations included in their book went well beyond circumnutation, however, covering other plant movements, such as phototropism and heliotropism.

The Restless Plant

Fast-forward 130 years, and we have a delightful new book on the same subject.

The Restless Plant is by the late Dov Koller, who was Professor Emeritus in the Department of Botany at the Hebrew University of Jerusalem.

From Koller’s Preface to the book: “The Restless Plant is my comprehensive, up-to-date account of the incessant movements in all actively growing plants. Developments and advances in this area of plant sciences since the 1880 publication of Darwin’s The Power of Movement in Plants have been tremendous. This is not a textbook, nor is it a scientific review. It is intended for the general, educated lay public of all scientific and nonscientific disciplines as well as for that part of the general public who might be interested in the endless variety and fascinating phenomena of plant movements, and in what is known about what makes them happen.“

Unfortunately, Professor Koller died before completing this book, but, fortunately, Professor Liz Van Volkenburgh agreed to take on the partially completed manuscript and finish the job.

When plants experience environmental stress, such as very hot temperatures, interesting things may happen inside plant cells at the genetic level.

For instance, heat stress (typically, leaf temperatures above 95o F for several hours) may increase the activity of “jumping genes” within the plant genomes.

The scientific name for such mobile genetic elements is transposon, from the fact that these pieces of DNA can “transpose” or “jump” from one place within the plant cell’s genome to another location.

Since transposons insert themselves randomly within the genome, they may land inside a functional gene. This is somewhat like throwing a genetic “monkey wrench” into the functional gene, effectively rendering it non-functional, that is, causing a gene mutation.

(Interestingly, if the transposon “jumps” back out of the effected gene, then its normal function may be restored.)

The American geneticist Dr. Barbara McClintock first discovered and described the nature of transposons. For this she was awarded a Nobel Prize in 1983. (A brief summary of her discovery of transposons can be found here.)

Erasing “Bad” Memories?

Some have suggested that the fact that some transposons are activated by stress contributes to evolution (adaptation to stressful environments, for example) by helping to “stir the genetic pot”, so to speak (see Ref. 1 below, e.g.)

Another way of thinking about this that, if these genetic changes are passed onto the plant’s offspring, then this serves as sort of a trans-generational “memory” of environmental stress.

A recent paper (Ref. 2 below), however, provides evidence that plants may actually have mechanisms that suppress these “memories” by effectively “erasing” the new, stress-induced transposons (called retrotranspons) from the genome prior to sexual reproduction (i.e., flowering).

The gist of the paper is perhaps best expressed via the Editor’s Summary:“The transcription of repetitive elements such as retrotransposons — mobile genetic elements constituting more than 40% and 60% of the human and maize (corn) genomes, respectively — is normally repressed, to prevent their unchecked dissemination throughout the genome. Ito et al. show that heat stress in Arabidopsis plants induces transcription of the ONSEN retroelement. Accumulation of ONSEN is suppressed by small interfering RNAs (siRNAs). In the absence of siRNAs, new ONSEN insertions appear in the progeny, having transposed during differentiation. These results imply a memory of stress that is counteracted by siRNAs, providing a way of preventing transgenerational retrotransposition in plants facing environmental stress.”

Bottom Line: Plants may possess genetic mechanisms to accelerate evolution in response to changing environments, but they may also have “brakes” on such systems as well.

In the previous post I suggested that the Venus flytrap works something like a mousetrap. And I described how the “trap” is hydraulically set. (For a more thorough explanation of how the Venus flytrap snaps, please see PDF file here).

But how do the trigger-hairs on the surface of the flytrap’s leaves act to “spring the trap”?

This action potential travels to the midrib of the leaf where it promotes the opening of water channels called aquaporins. This facilitates the rapid water efflux from key cells that hydraulically control the leaf opening.

More simply put, the stimulation of the leaf hairs produce electrical signals that cause the rapid deflation of the water-pressurized cells that keep the leaves open. And, thus, the “trap is sprung”.

(Please see here and here for some recent information regarding the the kinetics and mechanism of the Venus flytrap.)

Electrical Signals in Plants?

Do plants have a nervous system?

The short answer is: no. (At least not the complex nervous system of animals.)

But scientists have been able to detect transient electrical signals somewhat analogous to action potentials under certain situations in plants.

Sparked by a correspondence with Darwin, which included some Venus flytrap plants, the English physiologist John Scott Burdon-Sanderson was the first to discover action potentials in plants following stimulation of a leaf. (Please see reference 1 below.)

Do Plants Have a Neural Net?

In addition to thigmosnastic plants, all vascular plants may be utilizing electrical signals to regulate a variety of physiological functions.

Many of the biochemical and cellular components of the neuromotoric system of animals has been found in plants. And this has led to the hypothesis that a simple neural network is present in plants, especially within phloem cells, which is responsible for the communication over long distances.

Bottom line: Though plants don’t have a nervous system like animals, plants do have the necessary electrical, biochemical, and cellular components indicative of a neural network, albeit a relatively simple one.

The professor teaching my Introductory Botany class at the time loathed this book. He actually stated in class that any student he caught in possession of this book would receive a grade of “F”. (True story!)

Although this prof’s reaction was perhaps an extreme example, this story serves to illustrate the general attitude within the scientific community against any suggestion that plants possess a nervous system .

The notion that plants could feel pain, for example, or move rapidly in response to stimuli was solely within the realm of science fiction. (see here and here, for example)

What About the Venus Flytrap?

There are, however, some examples of relatively rapid movements in some plants in response to external stimuli, the most famous of which is the Venus fly trap.

Another example is the rapid leaf movements in the touch-sensitive plant Mimosa pudica. (see a YouTube video here)

In this case, as well as in the Venus fly trap, it’s not so much that the plant is moving in response to mechanical stimulation, but that the touch is triggering a sort of spring-loaded mechanism. Think of an old-fashioned mouse trap – gently touch the triggering mechanism, and the trap snaps shut.

In these plants, however, it’s kind of a hydraulic spring-loading. That is, when some cells within a thickening at the base of the leaves called a pulvinus have a high turgor pressure, this causes the leaves to open. And if these cells lose turgor pressure, the leaves close.

But how does mechanical stimulation trigger the rapid loss of cell turgor pressure in these plants?

In plants, this is mainly generated by converting the chemical energy of ATP into electrochemical energy by proton pumps. (Your cells use Na/K-ATPases.)

Some of the energy in this membrane potential is used by cells to accumulate solutes such as sugars and mineral ions such as potassium. This accumulation of solutes draws water into the cells via osmosis.

This is how the pulvinus cells in the Venus fly trap and the touch-sensitive plants likely generate their turgor pressure to open the leaves.

Now the leaves are hydraulically “spring-loaded”, and ready….

Next time: How electrical signals “spring the trap”. Also, an introduction to the emerging field of Plant Neurobiology.

Floral odors, produced by the vast majority of flowering plants, play important roles in plant–pollinator interactions.

A recent report of an orchid that attracts pollinators with the smell of carrion (see reference 1 below) reminded me of the infamous Voodoo lily, which was the subject of one of the professors in my department when I was a grad student.

Before I get to the Voodoo lily (a.k.a. “corpse flower”), what’s the story about this orchid?

From the abstract of ref 1: “Although pollination of plants that attract flies by resembling their carrion brood and food sites has been reported in several angiosperm families, there has been very little work done on the level of specificity in carrion mimicry systems and the importance of plant cues in mediating such specialization.“

The authors, who are at the University of KwaZulu-Natal, South Africa, studied the orchid Satyrium pumilum, native to the dry inland regions of the southwest cape of South Africa, and a local assemblage of carrion flies that pollinated this plant.

Briefly, from the conclusion of this paper:“Satyrium pumilum selectively attracts flesh flies, probably because its relatively weak scent resembles that of the small carrion on which these flies predominate.“

I’ve previously posted about the Voodoo lily (Sauromatum guttatum) with regard to thermogenesis in plants. But I didn’t tell much about the awful smell this flower produces.

The odor of the flowering Voodoo lily is somewhat infamous. Imagine what hamburger would smell like if a package of it sat inside a car in the summer, for about a week. It’s really that bad.

Turns out that some of the volatile chemical constituents of the odor produced by the Voodoo lily are also produced by the stinkhorn mushroom (see ref 2 below).

Botanical Term of the Day: “Sapromyiophily”

Hundreds of individual plant species from at least eight plant families “…emit odours reminiscent of rotting fish, carrion or dung. These odours mimic the substrate to which insects within the orders Coleoptera and Diptera (Wiens, 1978; Faegri & Van der Pijl, 1979) are usually attracted in order to oviposit or feed. The attraction of flies to brood-site and food mimics has given this distinct, deceptive pollination syndrome its common name: sapromyiophily. Sapromyiophilous flowers present adaptations to their special method of pollinator attraction involving situation, shape, colour, pattern, texture, scent, thermogenesis, motile appendages and changes of posture (Proctor et al., 1996). The plant families involved are diverse, yet they show both clear parallels between families and, nevertheless, a high variation within families.” (from ref 3 below, which is, by the way, a good reference source)

Bottom line: What’s in a name? That which we call a “corpse flower”, by any other name would smell as sweet…to a carrion fly. (apologies to W. Shakespeare)

Contrary to Dr. Fedoroff’s statement on Science Friday, there is ample evidence that transgenes have “leaked” from GE crops to other plants.

For example, in a previous post regarding the herbicide Roundup® (glyphosate), I noted how a transgene conferring resistance to Roundup® had escaped from a test plot of genetically-engineered turfgrass to adjacent populations of a related native grass.

There certainly are other published examples (see ref. 1, e.g.) of gene flow from GE crops to other non-GE crops and to weedy or wild relatives. And, as genetically-engineered organisms (GEOs), such as crop plants, proliferate, there will likely be more.

But, although gene flow from GE plants to wild relatives has been well documented, the ecological significance of these occurrences is much less well understood.

“Overall, there are relatively few data available with which to evaluate the potential for increased weediness or invasiveness in a crop species with fitness-enhancing abiotic and biotic GM traits. A better understanding is needed of the factors that presently control population size and range limits of either the crop volunteers or wild recipient populations, and the degree that survival or reproduction in the field is presently affected by the relevant biotic or abiotic stress-tolerance trait.” – from Ref. 1 below.

In a sense, Dr. Fedoroff is correct in stating that this is a “management” issue. But perhaps such management of GE crops should be conducted primarily by plant ecologists, such as Dr. Snow (see ref. 2, e.g.), rather than by plant genetic engineers.

Re. DIY Biotechnologists: This is certainly one of the serious drawbacks of GEOs that all of you DIY genetic engineers must seriously consider before releasing your creations into the wild.

2. Snow, Allison A. (2010) “Risks of Environmental Releases of Synthetic GEOs”, Invited Presentation for the Presidential Commission for the Study of Bioethical Issues, July 8, 2010 (PDF) The agenda and video of this meeting are available here.

The first generation of transgenic plants was in its infancy in the 1980’s, came of age in the 1990’s and seems to have settled into staid middle age in the past ten years.

So, what’s next?

A succinct answer is provided here thanks to the Crop Science Society of America (CSSA): “First-generation genetically modified (GM) transgenic crops with novel traits have been grown in a number of countries since the 1990’s. Most of these crops had a single gene that allowed them to tolerate herbicide application, giving them an advantage over wild species.

Second-generation transgenic crops are now being tested in confined field trials around the world. Some of these traits will allow crops to tolerate environmental stress such as drought, cold, salt, heat, or flood. Other traits being developed may lead to increased yield or lower nutrient requirements, or increase tolerance to disease and pathogens.”

New generations of transgenic plants are being produced, as, perhaps, a way to counter potential “mid-life crises” of current GM crops.

One way to make the “next generation” of GM crop plants is to simply add two or more “commercially-desirable” traits, such as herbicide tolerance and insect resistance, to one plant.

According to the GMO Compass website: “Herbicide tolerance (HT) continues to be the most common transgenic trait in GM crops worldwide.” And “Insect resistance (mostly Bt) is the second most common genetically modified trait. Herbicide tolerance and insect resistance (Bt) often are introduced simultaneously to a crop in one transformation event. This is called trait stacking. The third most commonly grown transgenic crop was stacked insect resistant/herbicide tolerant maize. Combined herbicide and insect resistance was the fastest growing GM trait from 2004 to 2005, grown on over 6.5 million hectares in the US and Canada and comprising seven percent of the global biotech area.”

Another class of second generation GM plants is more complex phenotypically than “stacked” GM plants. Such “ambitious” phenotypes may result from the insertion of multiple genes – even artificial chromosomes called minichromosomes – into GM plants.

“Instead of attempting to generate useful transgenic plants by introducing single genes, we now see an increasing number of researchers embracing multigene transfer (MGT) as an approach to generate plants with more ambitious phenotypes. MGT allows researchers to achieve goals that were once impossible – the import of entire metabolic pathways, the expression of entire protein complexes, the development of transgenic crops simultaneously engineered to produce a spectrum of added-value compounds. The potential appears limitless.” (from reference 1 below)